Orthogonal Frequency Division Multiplexing (OFDM) has gained considerable interest in recent years. However, OFDM is extremely sensitive to carrier frequency offsets, which are mainly caused by the inherent instabilities of the carrier frequency oscillators of the transmitter and receiver. Further, symbol timing synchronization must be achieved within an acceptable preamble period. This is due to the fact that symbol-timing errors may cause inter symbol interference (ISI) since Fast Fourier Transformation (FFT) windows can include adjacent OFDM symbol components. To ensure ISI-free detection, precise timing information regarding where the symbol boundary lies is needed to ensure that an uncorrupted portion of the received OFDM symbol can be sampled for FFT.
In wireless communication systems, the so called Random Access Channel (RACH) is an uplink channel, which is always received from the entire cell. The RACH carries the messages of the user terminals (UT) identity number, capability class, etc to report to the base station (BS), and the command to require system configuration information from the BS. To ensure the synchronization to BS, the RACH usually carries a sequence of pilots as well.
The design of RACH typically relies heavily on the radio access strategy, e.g. physical (PHY) layer access technology, frame structure, synchronization method, etc. For OFDM-based mobile communication system, RACH may have more choices by considering the frequency-time domain characteristics of OFDM.
Referring to IEEE802.16-2004 standard, the initial ranging frame is inefficient due to two preamble OFDM symbols. Comparing to user-specific PRACH scrambling in WCDMA system, it is less effective due to single preamble pattern in the initial ranging frame. In addition, 802.16-2004 is for the fixed broadband wireless access system, whose initial ranging is not good for cellular systems.
However, the method of WCDMA RACH cannot be used for OFDM systems, because the scrambling code is generally not a mandatory requirement in OFDM systems.
Generally the RACH is a contention-based channel. The UL synchronization is based on the user-specified pilot sequence. A set of preset orthogonal pilot sequences is needed. In WCDMA system [1], the RACH is characterized by a collision risk and by being transmitted using open loop power control. IEEE802.16-2004 [2] also adopts the contention-based random access, called initial ranging, where a specific frame structure is used starting with a long preamble.
Therefore, there is a need for improved synchronization for OFDM-based communication systems.